1. Field of the Invention
The present invention relates to a ceramic catalyst body and a ceramic carrier used in a catalytic converter for purifying an exhaust gas of an automobile engine or the like, and to a method of producing the same.
2. Description of the Related Art
There have been increasing demands in recent years to reduce the CO2 emission from automobiles, through improvement in the combustion efficiency and by cleaning the exhaust gas, for the preservation of the global environment. Accordingly, three-way catalysts have been widely used as the catalytic converter for purifying an exhaust gas, and the use of a NOx occluding reduction catalyst or the like has been increasingly employed for Diesel engines. These catalysts are typically produced by coating the surface of a monolithic carrier, such as honeycomb structure made of cordierite which has high heat resistance, with a material that has large specific surface area such as γ-alumina and depositing a noble metal catalyst such as Pt or Rh to be supported on the ceramic carrier. The NOx occluding reduction catalyst has, in addition to the main catalyst component, an NOx occluding substance as a promoter which adsorbs NOx molecules which are released in lean atmosphere, and releases the NOx in rich atmosphere, so as to be reduced by the noble metal catalyst and so that NOx is detoxified.
The catalyst of the prior art has a coating layer of the material of large specific surface area, because the cordierite carrier of the prior art does not have a specific surface area large enough to carry the required amount of catalyst component. When the surfaces of the carrier is coated with the material of a large specific surface area, however, the heat capacity of the carrier increases due to the increase in the mass, which is undesirable in view of early activation of the catalyst. The coating method also has a problem in that the decrease in the opening area of the cell of the honeycomb structure leads to an increase in the pressure loss, and the thermal expansion coefficient becomes larger than that of a carrier which is made solely of cordierite.
A ceramic carrier which can support a required amount of catalyst component without forming a coating layer by increasing the specific surface area of cordierite itself has been already known (for example, Japanese Patent Examined Publication No. 5-50338). However, the ceramic carrier is not practically available because of its short, catalytic life. It was found that evaporation of the catalyst during operation over a long period of time might decrease the catalyst performance. This is because the noble metal used as the catalyst turns into an oxide with a low melting point through contact with an oxidizing gas. It was found that the catalyst described above is more prone to the problem of catalyst evaporation than the catalyst of the prior art which supports the catalyst in pores of γ-alumina.
Also, it was found that this ceramic carrier is vulnerable to the influence of the exhaust gas, which flows through the cells because the catalyst is supported directly in a multitude of pores which open to the surfaces of the cell walls, and is therefore more liable to damage caused by catalyst poisoning than the catalyst of the prior art which supports the catalyst in the porous coating layer. An alkali metal or an alkali earth metal used as the NOx occluding substance is turned into a sulfate by poisoning with the sulfur which is included in the exhaust gas and, as a result, it loses the NOx occluding capacity. In the three-way catalyst, there is also a possibility that a trace component, which is added as the promoter, is damaged.